Jupiter’s hot spots: Quantitative assessment of the retrieval capabilities of future IR spectro-imagers

Abstract

Jupiter’s atmosphere presents limited regions of relatively thin cloud coverage (the so-called ‘hot spots’), which allow thermal radiation by warmer, deeper atmospheric layers to be transmitted directly to space. Hot spots therefore represent a means for probing physical conditions (namely chemical composition) below the main aerosol deck. Forthcoming missions to the Jovian system – Juno and EJSM spacecrafts – will host as payload components spectro-imagers operating in the infrared. Their coverage of 5 μm CH 4 transparency windows make them particularly suitable for the investigation of hot spots. This study is an assessment of their retrieval capabilities on the evaluation of gaseous mixing ratios from nighttime observations, on the basis of Bayesian theory. The retrieval performance is evaluated for the JIRAM instrument, a confirmed payload component of Juno. Its data will provide effective constraints on the mixing ratios of water vapor between 40 and 70 km below the reference 1 bar pressure level (between 3.5 and 7 bars). Assuming an a priori correlation length equal to half the scale height, we achieve a minimum retrieval uncertainty of 0.17, once the mixing ratio is given in terms of log 10( α), with α being the adimensional mixing ratio (vs. altitude) relative to a given reference profile. The JIRAM-Juno dataset will further allow determination of the ammonia mixing ratio, with a minimum relative retrieval uncertainty of 0.32 in the same altitude range, and of the phosphine mixing ratio, with comparable uncertainty up to the reference altitude. The retrieval performance is evaluated for a second instrument VIRHIS, which is a proposed payload component of Jupiter Ganymede Orbiter (JGO), one of the two spacecrafts of Europa-Jupiter System Mission (EJSM). This instrument has the benefit of higher spectral resolution and extended spectral range, when compared to JIRAM-Juno. Evaluation of the water vapor retrieval shows the uncertainty would be reduced to 0.08 with VIRHIS. The ammonia retrieval range would be expanded up to 10 km (0.66 bar), with a minimum uncertainty value of 0.10. Both instruments will place these measurements in a spatial context due to their simultaneous imaging capabilities, enabling therefore a number of studies covering chemical and dynamical aspects of atmospheric evolution.